United Kingdom Patent Specification No. 967,329 discloses placing the flange at the bottom of a railway rail in two channel members, each comprising a single moulding of resilient and electrically insulating material which can be regarded as consisting of a pad portion, on which the flange of the rail rests, and two upstanding side wall portions extending upwardly from it to locate the flange of the rail and electrically insulate it from side walls of a recess in a concrete railway sleeper in which the channel members are inserted. If the pad portions are sufficiently soft, resilient and yieldable as to keep to an acceptable level the transmission of vertical impact forces from a passing train to the sleeper, the side wall portions, being of the same material as the pad portions, will be unable to resist satisfactorily the lateral forces exerted on them by the edge of the rail flange and the rail will move too far sideways as the train passes and too much chafing of the channel members will occur.
United Kingdom Patent Specification No. 2,114,635A discloses placing the flange at the bottom of a railway rail in a channel member comprising a single moulding of resilient and electrically insulating material which can again be regarded as consisting of a pad portion, on which the rail rests, and upstanding side wall portions of the same material extending upwardly from it, the side wall portions having metallic reinforcing bars embedded in them and extending along them. The bars are intended to overcome the above-mentioned disadvantage of the channel members of Specification No. 967,329 but the presence of the bars increases the cost and difficulty in making the channel member.
United Kingdom Patent Specification No. 1,427,974 discloses placing the flange of a rail in a channel member which is stated to be of "a suitable thermosetting or thermoplastic plastics material for example a phenolic resin, or polyurethane", high density polyethylene being given as an example. Since it is a channel member, it can be regarded as comprising an under-rail portion and two upstanding side wall portions extending upwardly from it to locate the rail. There is placed on the under-rail portion, between the side wall portions, a rubber liner or pad which does not have any upstanding portions and is stated to be "more resistant to physical wear, because high insulating properties are not required of it", which presumably means that the rubber pad or liner is more resistant to physical wear than is the channel member, although whether or not the rubber is of a grade which is as stiff as that of the channel member and whether or not the material of the liner or pad is such that it would normally be called an electrically insulating material in the art of electrically insulating railway rails is not made clear in the specification. It is known, however, that the invention has been exploited by putting on the market a stiff channel member with a less stiff or softer rubber liner or pad laid on the under-rail portion of the channel member. If the liner or pad is thin, there is not enough cushioning for the rail with regard to vertical impact forces and if the liner or pad is thick the total thickness of the material between the concrete and the bottom of the rail flange is too great; in other words the recess in the top of the concrete sleeper to receive this material is unacceptably deep so that the sleeper is weakened to an unacceptable degree.